Apparatus for creating by induction an electric discharge in a gas at low pressure



Nov. 22, 1960 P HUBERT 2,961,557 APPARATUS FOR CREATING BY mnuc'rxon ANELECTRIC mscmncs IN A ens AT LOW PRESSURE Filed June 9, 1958 2Sheets-Sheet -1 Nov. 22, 1960 P. HUBERT 61,557

APPARATUS FOR CRE NG BY INDUCTION AN ELECTRI DISCHARGE A GAS AT LOWPRESSURE Filed June 9. 1958 2 Sheets-Sheet 2 2,961,557 'Patented Nov.22, 196i) APPARATUS FOR CREATING BY INDUCTION AN ELECTRIC DISCHARGE IN AGAS AT LOW PRESSURE Pierre Hubert, Sceaux, France, assignor toCommissariat a lEnergie Atomique, Paris, France, a state administrationof France Filed June 9, 1958, Ser. No. 740,644

Claims priority, application France June 12, 1957 11 Claims. (Cl.313-62) The invention, relates to apparatus for creating byelectromagnetic induction a high discharge current in a gas containedunder low pressure in a tubular chamber closed upon itself, that is tosay of doughnut shape, and preferably of toroidal shape.

The object of the present invention is to improve the stabilizing ofthis discharge in such a manner as to prevent any contact of the ionizedgas or plasma with the walls of the chamber, which contact would resultin a soiling of the plasma by evaporation of the material of which saidwalls are made.

Such a stabilizing permits of reducing the causes of cooling of theplasma and consequently of increasing the temperature obtained therein.It is known that the production of such a discharge constitutes one ofthe means for obtaining a controlled thermonuclear fusion in order tosupply a considerable amount of energy.

Various apparatus have already been imagined to create a discharge suchas above mentioned and to confine it at a distance from the walls of theannular chamber.

Among these apparatus, may be cited the following ones:

The Smith apparatus (Physical Review, 1947, 71, 135), which makes use ofthe stabilizing properties of a metal lic tore surrounding thedischarge, said tore being provided with a radial insulating cut toprevent shortcircuiting the driving electro-motive force.

The Bostick et al. apparatus experimented at Tufts College (Nucleonics,vol. 14, No. 2, 1956, page 44) in which an auxiliary winding wound abouta tore creates therein a homogeneous magnetic field, but for otherpurposes than stabilization of a discharge, for instance to study theoscillations of a plasma (Physical Review, 97, No. L, 1955, page 13).

The apparatus known under the name of Perhapsatron (Nucleonics, vol. 15,1957, page 108) in which the inductor winding is made of insulatingturns disposed in parallel along planes parallel to the center line ofthe tore.

The Cousins et al. apparatus (Proceeding Physical Society, 1951, 64 B,page 159), in which the inductor winding is constituted by a metallictore surrounding the discharge.

Each of these apparatus has drawback such as the following ones:

Imperfect stabilization of the discharge,

Defective efficiency of the discharge due to the high distance betweenthe inductor circuit and the induced circuit (Smith, Bostick or to adefective adaptation to the impedance between the source and theinductor circuit (Cousins),

Lack of regularity of the magnetic field in the vincinity of the zonewhere the current lead-in conductors are connected to the inductorwinding (devices such as the perhapsatron in which the casing of thechamber is not metallic),

Lack of gastightness of the chamber (in the case of apparatus in whichonly the metallic envelope is provided to ensure said gastightness).

In order to obviate these drawbacks, according to the present invention,use is made of an annular discharge tube the wall of which isconstituted by, or preferably lined with, a sheath of a conductingmaterial provided with at least one insulating cut, along which wall iswound a stabilizing winding connected with a source of direct current,the turns of said winding being disposed substantially in planesperpendicular to the central line of said chamber.

Furthermore, the inductor winding is advantageously constituted by aconductor connected with a source capable of supplying within a veryshort time a high electrical energy, the turns made by said conductorbeing disposed substantially in planes parallel to the center line ofthe annular chamber.

The shape of the tubular chamber in which the discharge is produced isnot critical. It is sufficient to have this chamber constituted by atube closed upon itself and which does not include portions that are toosharply curved. In the following description, this shape will be calleda doughnut shape.

Advantageously, this tube is of toroidal shape.

This tube is preferably made of a material which is gastight, is a goodinsulator, can be perfectly degassed and is as refractory as possible.In this respect the glass known under the trade-name of Pyrex, porcelainand vitrified silica are particularly suitable.

In addition to the gastightness it ensures, this chamber permits ofdistributing the potentials in the vicinity of the insulating cutprovided in the conductor sheath and therefore of preventing arcsbetween these lips.

Said sheath, disposed on the outside of the insulating wall of the tube(when such a tube is provided) conforms to the shape of this wall asclosely as possible.

In order to distribute in a more uniform manner the field in thevicinity of the cut in the sheath, said out is preferably covered by alocal annular strip of conducting material, for instance in the shape ofa portion of a metallic tore, electrically insulated from the conductorsheath.

Said conductor sheath may be made, for instance, by winding about theannular chamber a copper braid so as to form two crossed layersconnected by welds. It is also possible merely to use a suitable shapedmetal sheet, the metal that is chosen being then as good an electricalconductor as possible (aluminium, copper).

The whole of the annular chamber and of the conductor sheath may also bemade in the form of a metal tore the inner wall of which is enamelled.

The advantage of the conductor sheath of shield is that any deformationof the plasma filament causes the formation in said sheath of inducedcurrents which create a magnetic field exerting on the plasma a forcewhich opposes said deformation.

The stabilizing winding is constituted by insulated conductor wires theturns of which are wound regularly about the annular tube in planesperpendicular to the center line thereof.

Such a stabilizing winding creates a longitudinal magnetic fieldparallel at any point to the discharge, that is to say to the plasmafilament, which is confined along the central line of the chamber, dueto the pinch effect created by the electromagnetic attraction betweenparallel current lines.

If there is a deformation of this discharge, said field exerts thereonforces which oppose said deformation.

Theory teaches that the action of the magnetic field produced by thecurrents induced in the metal envelope chiefly opposes the deformationsthe wave length of which is higher than the length of the central lineof the chamber whereas the action of the magnetic field produced by thestabilizing winding chiefly opposes the deformations the wave length ofwhich is lower than the length of said central line.

Concerningth'e inductor winding, it is preferably constituted byinsulated conductor wires disposed regularly along the external surfaceof the-conductor sheath, the turns of this inductor winding beinglocated substantially in" planes'parallel to the plane of the centralline of the chamber and being advantageously disposed between the sheathand the stabilizing winding.

1 Said inductor winding consists of N elementary windingsconnected inparallel, each of these elementary windi'n'gs being constituted by aconductor forming it turns each adjoining the next one and covering acontinuous annular band'o'f the external surface of the conductorsheath.

'The number of turns it of each of said elementary windings is' chosenin such'm-anner as to adapt the source of current in the best conditionsto the impedance of the inductor winding. The'use of a plurality ofelementary windings connected in'parallel permits of preventingdeformations of the discharge having a wave length close to the lengthof the central line of the chamber (for instance contraction orexpansion of the whole of the discharge).

This result is due to the fact that if the discharge gets closer to oneof the elementary windings, there is produced, according to the Lenzlaw, an increase of the current flowing through said elementary winding,this increase having for its effect to oppose the displacement of thedischarge.

In these conditions, stabilization of the discharge is ensured by thecombined actions of the currents circulating respectively in theconductor sheath, in the stabilizing Winding and in the inductorwinding.

Theuse of a plurality of such elementary windings, as above described,is not quite necessary since the conductor sheath also serves to opposethis kind of deformation. But it is useful because it reduces theimportance of the part to be played by the sheath and therefore permitsof reducing its thickness.

The location of the inductor winding on the surface of the conductorsheath is preferred to thearrangement according to which the coupling ofthe discharge is ensured through a ferromagnetic core as in an ordinarytransformer.

r This is due to the following causes:

The natural self inductance of the discharge is reduced to aminimum,which permits of obtaining high current discharges without having tohandle too high a reactive energy.

The mutual inductance between the discharge and the inductor Winding ismaximum so that it is then possible to have a suflicient coupling evenin the absence of a magnetic circuit; one might thus consider thepossibility of working with a saturable magnetic circuit which would actonly during a fraction of the cycle of operation.

This arrangement permits of easily dealing with the considerableelectromagnetic forces which exist between conductors through which highcurrents are flowing: in the case of an annular chamber of toroidalshape, it can be demonstrated that the electromagnetic forces exerted onthe turns of the inductor winding by the field created during adischarge correspond to a pressure equal to the pressure existing in thedischarge divided by the ratio by the inductor winding. It is thereforepossible to concentrate the plasma under very high pressure without 4,having to deal with mechanical problems which are practically impossibleto solve.

The operation of the device is as follows:

When the voltage V of the source connected with the inductor winding issuddenly applied to the terminals of said winding, there is produced, inthe gas under low pressure contained in the chamber, an electromotiveforce which is substantially uniform and equal to When thiselectromotive force becomes suflicient, it produces a disruptivedischarge in the gas, through which a very high current may flow if'thevalue of V is sufiicient; the gas is then heated by the Houle effect toa very high temperature, which may permit of obtaining nuclear fusionsin this gas. The device works as a step-down transformer the primary ofwhich is the inductor winding and the secondary of which includes onlyone turn constituted bythe discharge itself.

' It is reminded that the'-pin'ch effect? may be suflicient by itself tokeep the discharge at a distance from the wall if the followingcondition is complied with:

where I is the current in amperes in the discharge,

P is the total number of electrically charged particles per centimeterof length of the discharge,

K is the Boltzmann constant, equal to l.380 l0- ergs/ C., and

T is the absolute temperature of the plasma in centigrade degrees."

The arrangements according to the present invention complete thestabilizing effect,'in particular for insufficient values of I.

Thus the longitudinal field created by the stabilizing winding and alsothe fields due to the current circulating through the metal casing bothhave, in addition to their stabilizing action, the property of slowingdown the diffusion of plasma toward the wall of the chamber when theelectromotive force which'produces the pinch effect is reduced to Zero.

The presence of these fields therefore makes possible a continuousoperation of the apparatus in which the inductor winding would be fedfrom a source of reciprocating current provided that the frequency ofthis current is suificiently high, that is to say that the portion ofevery cycle during which the current in the discharge is zero orpractically so (duning which portion of the cycle there is noconfinement due to'the pinch effect) is sufficiently short.

With such a reciprocating current feed, the pinch effect confinement ispossible permanently if condition (I) is complied with as an averageduring every cycle, that is to say if:

100/2 PK T (11) 1e being the effective current developed during thedischarge.

It should be noted that the total longitudinal field existing betweenthe plasma and the wall of the chamber may in some cases have adirection opposed to that created separately by the stabilizing winding,since the field due to the current induced in the metallic casing mayhave in some conditions a preponderating value: only the sum of thesetwo effects is to be considered.

In some cases, a magnetic circuit increasing the mutual inductancebetween the inductor winding and the dischargemay have afavorableaction. This circuit might for instance be constituted by a'soft ironcore surrounding at least a portion of the chamber, in a manner"analogous to the cores on which are wound the conventional inductorwindings (Bostick et al.).

The plasma, maintained by a system of electromagnetic forces, may be theseat of oscillations, either transient or not, giving rise toalternative compressions and expansions, for instance as a consequenceof a tendency to instability. These oscillations induce in the inductorwinding (or in another suitable winding) an electro- Emotive force whichmay be used to contribute in the heating of the plasma or to extractenergy therefrom.

The apparatus according to the present invention has many advantagesover those used up to this time to heat very rarefied gases to very hightemperatures, and in particular:

A stabilizing of the deformations for all wavelengths of the discharge;

An excellent efficiency due to the fact that the inductor winding andthe discharge are close to each other and also to the possibility ofadaptation between the impedances of this winding and of the inductorsource;

A homogeneous distribution of the field in the vicinity of the currentinlet and of the cut;

A good gastightness of the chamber;

The possibility of obtaining a very high current discharge whileproducing the inductor current with a conventional machine, since thecurrent in the discharge may be n times higher than that supplied by thesource (as it is known in transformers) which further reduces the lossesin the feed conductors;

The possibility of a continuous operation.

It shouid be noted that the fact of making the inductor winding in theform of a plurality of elementary windings as above indicated, may play,due to the stabilizing efiect it involves, a very important part in thecase of discharges of very long duration (for instance .averaging of asecond) for which it would not be possible to give the metal envelope athickness corresponding to the skin thickness. This case might occurwhen :a temperature higher than thirty millions of degrees is obtained,for which the conductivity of the plasma will be higher than that of allknown metals and in particular than that of the metal of which saidenvelope is made. It is known that, in this case, the feed of thestabilizing winding should be cut a short time t before the beginning:of the discharge, the value of this time t, which depends upon thespeed of diffusion of the magnetic field through the metal envelope,being at most equal to In this formula, e is the thickness of the metalsheath and p the resistivity of the metal thereof in ohms percentimeter.

A preferred embodiment of the present invention will be hereinafterdescribed with reference to the accompanying drawing given merely by wayof example and in which:

Fig. 1 diagrammatically shows a discharge according to the invention,and

Fig. 2 is a diametral vertical section of this apparatus with parts incross section.

On this drawing, only portions of the winding, respectively designatedby reference numerals 1 (inductor winding) and 2 (stabilizing winding)have been shown.

The annular chamber 3 is constituted by a toroidal envelope of Pyrexglass having a mean diameter of 78 cuts, the diameter of the circle therotation of which forms the tore being equal to 8 cms.

The inside of this chamber is connected with a pumping conduit 4 of adiameter equal to 1.5 cm. itself connected with a pump capable ofproducing a vacuum ranging from 3 10- to 10" mm. of mercury.

This chamber 3 is covered with a conductor shield 5 made of a copperbraid provided with a cut 6 the width (about 1 centimeter) of whichcorresponds substantially to one tenth of the diameter of the diameterof the above mentioned circle.

This cut is covered by a metal strip 7 insulated from shield 5.

The inductor winding 1 includes six conductors eadh constituted by acopper wire of 2 mm. diameter enclosed in a sheath of polythene of adiameter of about 1 cm. connected in parallel with the current source 8and each of which makes four turns about the tore. The four turns ofevery conductor are adjacent to each other and occupy the portion of theexternal surface of the tore corresponding to an angle of 60.

The stabilizing winding 2 is constituted by four superimposed layers ofenamelled copper wire of a diameter of 2 mm. These layers are wound insuch manner that the respective magnetic fields they produce are addedto one another but that the sum of the mutual inductance of one layerwith the inductor winding and of the mutual inductance of the next layerwith said inductor winding is practically zero. For this purpose, itsuffices, in the example shown to wind wire 2 always in the samedirection; for instance, supposing that the center line of chamber 3(Le. the circle on which are located the centers of all the circularcross sections of said chamber) is located in a horizontal plane, eachturn of each layer is wound so that it runs upwardly, from below saidplane to above it, on the outer portion of said turn and downwardly,from above said plane to below it, on the inner portion of said turn.

The envelope constituted by the stabilizing winding 2 is provided with acut analogous to that provided in shield 5. This cut must constitute anelectrical insulation capable of supporting between its edges apotential difference equal (in this case 1L 4 induced by winding 1.

The source of current 8 includes a bank of condensers of 50 kv. and 60microfarads fed by a high voltage generator. Its discharge is controlledby spark gap 11 having three electrodes and it is transmitted to theelementary inductor winding 1 through six coaxial cables 12 mounted inparallel, which ensures a better resistance to mechanical efforts andreduces the importance of the parasitic self-inductances due to themounting.

A direct current source 13, constituted by a rotary machine of 600 v.and A. of usual type supplies current to the stabilizing Winding 2.

An observation window is constituted by a mere orifice of 5X20 mm.provided in shield 7 opposite the cuts, this window extending betweenthe wires of winding 1 which are slightly spaced apart for this purpose.

In order to facilitate starting, there is provided an induction coil 14capable of exciting an electrode 15 surrounding on the outside thepumping conduit 4, thus ionizing some particles of the gas circulatingthrough this conduit.

For this purpose, use may be made of an auxiliary discharge establishedbetween electrodes passing through the wall of chamber 3, or also ofelectron-guns.

A pumping system 16 connected with conduit 4 permits of evacuatingchamber 3, of degassing it and of introducing the desired gas thereinto.

In order to measure the discharge current, it is advantageous to makeuse of two current transformers one of which 17 surrounds the coaxialcables 12 and is therefore sensitive only to the intensity of thecurrent in the inductor winding, Whereas the other one 18 surrounds aportion of the tore and is sensitive both to this intensity and to thatof the current flowing through the discharge. This transformercooperates with a suitable measurement device 19.

In a general manner, while I have, in the above de scription, disclosedwhat I deem to be a practical and efiicient embodiment of my invention,it should be well understood that I do not wish to be limited thereto asthere might be changes made in the arrangement, disposition and form theparts without departing from the principle of the present invention ascomprehended Within the scope of the accompanying claims.

What I claim is:

1. An apparatus which comprises, in combination, a closeddoughnut-shaped discharge tube containing a low pressure atmosphere ofan ionizable gas, means for inducing a high discharge current to flow insaid tube, a sheath of a conducting material closely surrounding thespace inside said tube, said sheath being doughnut-shaped except for atransverse out between the ends thereof located opposite each other, aband of an insulating material filling said cut, a stabilizing windingsurrounding said tube, each turn of said stabilizing winding beinglocated in a plane substantially at right angles to the portion of thecenter line of said tube surrounded by said turn, and means for passingdirect current through said'stabilizing winding, said current inducingmeans being independent of said sheath of conducting material.

2. An apparatus which comprises, in combination, a closeddoughnut-shaped discharge tube containing a low pressure atmosphere ofan ionizable gas, a sheath of a conducting material closely surroundingthe space inside said tube, said sheath being doughnut-shaped except fora transverse cut between the ends thereof located opposite each other, aband of an insulating material filling said cut, at least one inductorwinding wound on said sheath, the turns of said winding being located inrespective planes parallel to the center line of said tube, a sourcecapable "of releasing a high electrical energy within a short time, theterminals of said source being connected with those of said windingrespectively, a stabilizing winding surrounding said sheath, each turnof said stabilizing winding being located in a plane substantially atright angles to the portion of the center line of said tube surroundedby said turn, and means for passing direct current through saidstabilizing winding.

3. An apparatus according to claim 2 including a plurality of suchinductor windings disposed side by side on said sheath, said inductorwindings being connected in parallel with said source, each of saidinductor windings including a conductor forming a plurality of turnslocated side by side, all of said respective inductor windings havingthe same number of turns, the whole of the turns of said plurality ofinductor windings covering the whole external surface of said sheath.

4. An apparatus according to claim 1, in which said discharge tube ismade of a gas-tight insulating material and said sheath is mounted onthe outer wall of said tube.

5. An apparatus according to claim 1, further including an annular stripof a conducting material mounted around the ends of said sheath so as tosurroundfsaid band of insulating material," said' strip being insulatedfrom said sheath.

6. An apparatus according to claim 2, in which said stabilizing windingincludes an even number of superirnposer layers of turns of a conductor,said layers being wound so that the respective magnetic fields theyproduce are added to one another but that the sum of the mutualinductance of one of said layer with said inductor winding and of themutual inductance of the next of said layers with said inductor windingis practically zero, said layers stopping short of said cut in saidsheath, on both sides of said cut.

7. An apparatus according to claim 2, further including means formeasuring the discharge current, said means including a currenttransformer surrounding one of the terminals of said source and acurrent transformer surrounding a portion of said discharge tube.

8. An apparatus according to claim 2 in which said source is a highfrequency alternating current source.

9. An apparatus which comprises, in combination, a closeddoughnut-shaped discharge tube containing a low pressure atmosphere ofan ionizable gas, means for inducing a high discharge current to flow insaid tube, a sheath of a conducting material closely surrounding thespace inside said tube, said sheath 'being continuous except for atransverse cut between the ends thereof, a band of an insulatingmaterial filling said cut, a stabilizing winding surrounding said tube,each turn of said stabilizing winding being located in a planesubstantially at right angles to the portion of the center line of saidtube surrounded by said turn, and means for passing direct currentthrough said stabilizing winding, said current inducing means beingindependent of said sheath of conducting material.

10. An apparatus according to claim 9 having inductor windings parallelto the axis of said tube and on said sheath, said windings disposed sideby side on said sheath and connected in parallel with said source.

11. The apparatus according to claim 10 wherein said inductor windingscomprise a conductor forming a plurality of turns located side by side,all of said inductor windings having the same number of turns, the wholeof the turns of said plurality of inductor windings covering the wholeexternal surface of said sheath.

References Cited in the file of this patent UNITED STATES PATENTS2,627,552 Gurewitsch Feb. 3, 1953 2,640,923 Pollock June 2, 19532,658,999 Farley Nov. 10, 1953 2,683,216 Wideroe July 6, 1954 2,790,902Wright Apr. 30, 1957

